1. Field of the Invention
The invention relates to filtration in general and in particular to an improved apparatus and method for cleaning the intake air to be used in a gas turbine.
2. Description of the Related Art
A gas turbine typically requires large quantities of intake air for the combustion of fuel. The natural pollution of the surrounding air results in a considerable impairment of the turbine performance. For example, particles from the air deposit on the compressor blades of the gas turbine and lead to an imbalance or effect a change in the flow profile of the compressor blades. Salt crystals or salt aerosols contained in the air lead to corrosion in the gas turbine, in particular on the turbine blades. These and other mechanisms lead to a strong reduction of the turbine performance and energy efficiency. For this reason, the intake air is filtered to remove as much of the undesired contaminants as possible. Usually this is done by a larger number of filters which are located in a filter house. The filter house is connected to the gas turbine via a flow channel through which the purified air is fed to the gas turbine.
The filters are attached to at least one filter wall which constitutes within the filter house the partition of a relative raw gas area and a relative clean gas area in relation to the respective filter wall. The filter can also be called a matrix. The filter wall is sealingly connected to the walls of the filter house and is limited by them with regard to its height and width. The filter wall has openings through which the air exchange between the raw gas and the clean gas side can take place. The filters are installed in front of or in these openings.
The filters can have different designs. For example, filter cartridges, pocket filters and cartridge filters in all different embodiments are used. It is also not uncommon that several filter walls are positioned one behind the other in a series connection whereby the filter class and the separation efficiency of the filter is generally chosen in such a way that it increases from the first to the last filter wall. The filter wall positioned on the inlet side thus functions as a pre-filtration of the air for the subsequent filtration stages.
All current filtration systems have in common that the individual filters are only installed on one side of the partition wall. Usually this is the respective raw gas or dirty side of the filter walls. It is commonly considered advantageous to also deinstall the loaded filters at a standstill of the gas turbine from the raw gas side so that any contaminants coming lose from the filters cannot penetrate the clean gas area. The filters of the last filter stage in the flow direction, however, are always installed on the clean gas side of the filter wall.
Gas turbines are also used on offshore oil and gas production platforms. They are used to generate electricity for operating the platform or pumping and compressing, respectively, the produced oil and gas. Due to the confined space and the weight limitations of the platform structures, the filter houses are being built smaller and more compact as is usual in comparison to onshore installations. Due to the smaller dimensions of the filter houses, the dimensions of the filter walls are consequently also smaller and typically significantly fewer filters are installed as would be usual with the same turbine model onshore. Conversely, this means that the filters are operated with a considerably higher volume flow of the air to be filtered than onshore as the total volume flow is determined by the gas turbine. Accordingly, gas turbines operated offshore usually have a volume flow of approx. 7000 to over 8000 m3/h per filter whereas gas turbines operated onshore usually have a volume flow of 3400 to 4300 m3/h.
With regard to gas turbines operated onshore, there has been a trend to a higher-value filtration in recent years. In particular, the use of EPA or HEPA filters according to EN1822:2009 has proven to be advantageous as the effect on the energy efficiency of the gas turbine over the service life of the filter is very positive. An improvement of the filter medium's filtration efficiency, however, in general also increases its differential pressure. The differential pressure of a new filter is thus inter alia a function of the filter medium and the volume flow applied. As the maximum differential pressure of a filter house or the gas turbine itself is limited, a predetermined number of filters installed results in a limitation of the filter class. Due to the high volume flow of offshore gas turbines, filters according to EN779:2012 or equivalent are almost exclusively used. Higher-value filters according to EN1822:2009 or equivalent have not yet become prevalent although their positive effect on turbine efficiency would be desirable. In case of land-based gas turbines, it is possible to reduce the volume flow per filter by enlarging the filter house and consequently increasing the number of filters. In case of offshore installations, however, this is not possible in most of the cases. The installation space, in particular with regard to existing platforms, is simply limited. For example the height of each deck on an offshore platform is fixed and a filter house cannot be expanded across several decks. Multiple processes are tightly packed together with piping and cable ducts filling up most of the space in-between. Increasing the space of a filter house would therefore require a complete redesign and re-arrangement of multiple processes and would be a major undertaking, if not even impossible.
Another problem is the service live of filters. This is also a function of the volume flow in relation to the total deployed media area of a particular grade within a filter house. A longer service life of the filters is desirable in that it extends the intervals in which the filters have to be exchanged and thus increases the availability of the gas turbine. In the early phase of the filter life, the filter differential pressure changes little in response to rapidly changing environmental conditions, but towards the end of the filter life a relatively small change in e.g. humidity will result in a large and rapid increase in the differential pressure of the filter, which can trigger the alarm or trip limit of the turbine, leading to an unexpected shut down of the whole system. It is therefore desirable to change the filters before they enter the unstable condition and to avoid unexpected shut downs. Longer filter life not only reduces the ratio of unstable to stable operating condition of the filters, it also allows for a filter change before the unstable phase is entered. Thus the filter system will deliver better process reliability by increasing the deployed filter media area.
The invention has for its object to increase the number of filters in parallel order on a partition wall within a flow channel without having to enlarge the dimensions or cross section of the flow channel or partition wall.